While it has been established that stressors have immune-modulatory effects and may have health consequences, the mechanisms by which these occur is not well understood. This study will attempt to differentiate the roles of the dorsal and ventral aspects of the periaqueductal gray (PAG) in immune modulation. This study will utilize electrical and neurochemical site-specific stimulation of the PAG in rats and examine immune function in several immune relevant compartments. It is hypothesized that the PAG stimulation will show site specificity such that the dorsal and ventral aspects will differentially effect various compartments of NK cell function. Specifically, the dorsal PAG stimulation will effect peripheral blood NK functioning while the ventral PAG stimulation will effect splenic NK functioning. The biological relevance of these changes will be examined by an in vivo tumor induction model. This study will begin to examine the neurochemical mediators of the stimulation induced immune changes. It is hypothesized that the ventral PAG induced immune changes will be opiate mediated while the dorsal PAG effects may be due to other neurotransmitter systems such as the adrenergic or serotonergic.